Steve is correct except for one minor thing - you can't always go by watching the gauge pressures initially, you need to look at the cover of your pressure switch and see what the cut-in and cut-out pressures are supposed to be for your switch, then do as he says and drain all water pressure and set the air pressure in the bladder 2 psi lower than the cut-in pressure to get you in the ballpark. Then you can either fine-tune the air pressure or the switch itself after. I just had to adjust mine yesterday after being in service for 4 years and was getting screwy readings initially - my switch is basic at 30 - 50 and I know it was set at this (+/- 1 psi) 4 years ago when put in service new and my air pressure was set exactly at 28 psi. Has worked fine up until about a week ago when I noticed some "slamming". When I first looked at the cycling yesterday it was cutting in at about 42 psi (+/-) with a momentary huge drop and cutting out at about 54 psi (+/-). Luckily I drained water pressure and checked the air before trying to adjust the switch as I initially thought I would have to do - the air pressure was right about 41 or 42 psi !!! I don't know how the pressure increased over the 4 years but it surely did. I realize temp will make a small difference at times but 14 psi change was a little much, especially since the original set was in summer and it is now spring and much cooler. Anyway, after adjusting the air pressure back to 28 psi the switch was operating exactly as it was supposed to again without any adjustment at all. If I had started trying to adjust the switch first like I was considering, it would have been a real pain. I realize that most problems with an air bladder are going to be lack of air pressure so my situation may be pretty unique but just wanted to show that weird things can happen. Anyone got any idea how the pressure increased on me like that ?